A data merging method for merging valid data of one logical block in a rewritable non-volatile memory module is provided. The method includes assigning a plurality of log physical blocks for the logical block. The method also includes performing a data arrangement operation and a data move operation with a partial synchronization manner to copy the valid data of the logical block into the lower physical pages of the log physical blocks from a first data physical block and at least one spare physical block while programming the valid data of the logical block into a second data physical block from the lower physical pages of the log physical blocks in units of each physical page group. The method further includes remapping the logical block to the second physical block. Accordingly, the method can effectively shorten the time of merging valid data and improving the reliability of data writing.